Day navigator widget: setDates()
- change logic of setting mCurDate: if it's outside the conference range, set it to mStartDate
(and not to mEndDate when it's "greater") -- when going to an earlier
conference, starting on the last day doesn't really make sense
- update() the widget after changing dates.
this might be a bit expensive but it ensure that the displayed date is
what we want, and since there are many day navigator widgets there's
probably no single other place

Removed the ability to show "pictures" (maps) of rooms and maps of conferences.
The XML file does not contain picture/map/image information of conferences or rooms.
We left the room.picture definition in the database SQL because there is no "drop column"
in sqlite.